1. Field of the Invention
The invention concerns a method for producing a two-layer asphaltic surfacing with which an optimisation of the properties of the asphalt layers is effected.
2. Brief Description of the Background of the Invention Including Prior Art
As set forth by the methods known in the state of the art, multi-layer asphalt surfacings are produced using different types of asphaltic mixtures. The mixture is laid in courses or--if the same mixtures are worked--in layers on top of one another. To bond the layers, asphaltic emulsions are spread on the cold supporting medium.
Asphalt surfacings are used on roads as well as for other traffic surfaces. The total thickness of the structure and of the individual layers depends primarily on the prevailing loads. Generally, asphalt surfacings consist of surface, binder, and base courses. Individual courses must fulfil different requirements. For the surface course, primarily the following requirements are important:
for traffic safety reasons good grip, even surface, luminosity, and the highest possible reflectivity are important; PA1 to guarantee high weather resistance sealing, frost resistance, volume stability, adhesion of the binder, and oxidation stability are required and PA1 to ensure sufficient strength flexibility, stability and wear resistance, as well as fatigue strength requirements must be fulfilled.
Today, with the loads from heavy traffic, the binder course has the primary task of absorbing the shear stress without damage and diverting it. For this reason, stability must take priority in binder courses.
The asphalt base course serves to absorb, without damage, and divert the stresses occurring in the overlying areas. They must also be structured so as to be stable.
Binder and base course are protected by the surface course and therefore do not have any sealing tasks.
Generally, the surface courses are laid to a thickness of 4 cm. The nominal thicknesses of the binder and asphalt base courses vary depending on the building regulations applying to the construction class, such as RStO 86/89. Arand et al.: Investigation of the Efficiency of High Performance Compaction Beams Through Field Measurements on Rolling Construction Sites, Road and Motorways, Bonn 41 (1990) No. 5, pp. 215-219.
The evolution of asphalt road construction in the last decades was characterised by optimisation of the individual layers as regards composition, course thickness, and compaction, in order to meet the continuously growing loads from the growth in traffic load figures and the rise in axle loads. The influence of the axle loads upon working behaviour is generally determined pursuant to the results of the AASHO ROAD Test. This stipulates that fatigue and deformation damage increase approximately by the power of four of the axle load.
Specifically increasing axle loads as well as overloading and growth in heavy traffic through the continual increase in transport movement on roads has lead to an increase in the frequency of damage. Moreover, the evolution of road construction technology mostly lags behind the evolution of requirements. Involved in the damage caused by deformation are the surface, and partially also the binder, through to the base courses. The causes lie above all in material composition and compaction. The surface courses represent a special problem, especially since the multiplicity of tasks demand quite different conceptional decisions. For example, sealing and stability operate in a contradictory manner.
As a consequence of its sealing function, the surface course is the course richest in binder, which at the same time is subject to the highest possible temperatures. Owing to the thermoplastic properties of the bitumen, the surface course material has the lowest shear strength in summer.
A further problem, which is of immense importance for asphalt road construction, is laying asphalt under unfavourable laying conditions. This results in, in the case of the surface and binder courses of 4 cm layer thickness that frequently have to be laid, too little compaction time and too rapid cooling of the asphaltic mixture. Whilst having the same compaction expenditure vis-a-vis thicker laid thicknesses, lower degrees of compaction are achieved at lower thicknesses as a result of faster cooling.
In the state of the art it is known to improve the compactability by increasing the binder content. However, this reduces the shear strength. Generally, a lower degree of the compaction involves an exponential reduction in, for example, stability. The laying of single course of asphalt common in the state of the art, in which the next course is laid upon a cold or to a large extent already cooled supporting medium, results in constraints as well as quality deficiencies specifically in the case of these relatively thin surface and binder courses.
It is known, in the rehabilitation of asphaltic surfacings, to create two-layer asphaltic surfacings by heating the existing, mostly damaged asphalt and distributing it on the job site or mixing it with other asphalt components. Owing to the relatively low thermal conductivity of the asphalt, the bitumen is subject to widely differing temperatures during heating.
On the top side of the asphalt layer temperatures of 250-600.degree. C. occur which decrease sharply downwards. These high temperatures lead to greater environmentally harmful emissions as well as to changes in the properties of the bitumen.
Furthermore, the quality of the distributed or mixed components on the road is subject to large fluctuations compared with asphalt produced by the mixing process at an asphalt mix plant, since the ratio of old and new components is to be less adhered to and the contract section often contains different compositions.